Observed Clutch Size Research Articles

Northern Pygmy-Owl (Glaucidium gnoma) nesting ecology in northwestern Oregon

I report on the nesting ecology of the Northern Pygmy-Owl (Glaucidium gnoma) in northwestern Oregon based on 71 breeding attempts, including observations at 66 nest cavities during 12 years of study (2007–2013, 2016–2020). I focus on the annual shift in the start of laying and in productivity (clutch size) in relation to prey composition. For all years combined, the start of laying spanned 2 months (27 Mar–30 May). The mean annual start of laying varied across consecutive years by nearly 3 weeks (19.0 ± 6.5 d); mean productivity at nests varied annually by 23% (1.2 ± 0.5 eggs). Observed clutch size was most frequently 5 or 6 eggs (5.6 ± 0.9, range 4–8 eggs, n = 47), but most clutches in early-nesting years were 6 or 7 eggs, and 4 or 5 eggs in late-nesting years. The mean incubation and nestling periods were 29.4 ± 1.1 d and 26.4 ± 1.2 d, respectively. Pygmy-owls fledged a mean of 5.1 ± 1.1 young at 58 successful nests and 4.2 ± 2.2 for all breeding attempts. Vertebrate prey composition varied annually and seasonally. Pygmy-owls took mammalian prey more often early in the breeding season (66% of vertebrate prey) and in early-nesting years (65%); avian prey were more frequently taken late in the breeding season (66%) and in late-nesting years (66%). Pygmy-owls nested disproportionately in coniferous trees (χ2 = 67.48, P < 0.001, df = 1, n = 66); nests in western redcedars (Thuja plicata) fledged 1.5 more owlets and failed less often (5% failed) than nests in other tree species (22% failed). Observations at this largest ever collection of Northern Pygmy-Owl nests reveal a commitment by these small owls (1) to produce young annually, even when they had to overcome obstacles such as mate procurement, mate loss, and nest depredation, and (2) to use mammalian prey to increase productivity in some years.

Temporal changes in reproductive success and optimal breeding decisions in a long-distance migratory bird

Many avian migrants have not adjusted breeding phenology to climate warming resulting in negative consequences for their offspring. We studied seasonal changes in reproductive success of the greater snow goose (Anser caerulescens atlantica), a long-distance migrant. As the climate warms and plant phenology advances, the mismatch between the timing of gosling hatch and peak nutritive quality of plants will increase. We predicted that optimal laying date yielding highest reproductive success occurred earlier over time and that the seasonal decline in reproductive success increased. Over 25 years, reproductive success of early breeders increased by 42%, producing a steeper seasonal decline in reproductive success. The difference between the laying date producing highest reproductive success and the median laying date of the population increased, which suggests an increase in the selection pressure for that trait. Observed clutch size was lower than clutch size yielding the highest reproductive success for most laying dates. However, at the individual level, clutch size could still be optimal if the additional time required to acquire nutrients to lay extra eggs is compensated by a reduction in reproductive success due to a delayed laying date. Nonetheless, breeding phenology may not respond sufficiently to meet future environmental changes induced by warming temperatures.

Exploring patterns of variation in clutch size–density reaction norms in a wild passerine bird

Negative density dependence of clutch size is a ubiquitous characteristic of avian populations and is partly due to within-individual phenotypic plasticity. Yet, very little is known about the extent to which individuals differ in their degree of phenotypic plasticity, whether such variation has a genetic basis and whether level of plasticity can thus evolve in response to selection. Using 18years of data of a Dutch great tit population (Parus major), we show that females reduced clutch size with increasing population density (slopes of the reaction norms), differed strongly in their average clutch size (elevations of the reaction norms) at the population-mean density and that the latter variation was partly heritable. In contrast, we could not detect individual variation in phenotypic plasticity ('I×E'). Level of plasticity is thus not likely to evolve in response to selection in this population. Observed clutch sizes deviated more from the estimated individual reaction norms in certain years and densities, implying that the within-individual between-year variance (so-called residual variance) of clutch size was heterogeneous with respect to these factors. Given the observational nature of this study, experimental manipulation of density is now warranted to confirm the causality of the observed density effects. Our analyses demonstrate that failure to acknowledge this heterogeneity would have inflated the estimate of 'I×E' and led to misinterpretation of the data. This paper thereby emphasizes the fact that heterogeneity in residuals can provide biologically insightful information about the ecological processes underlying the data.

Postovulatory Follicles in Mallards: Implications for Estimates of Breeding Propensity

AbstractAbstractWe evaluated the use of postovulatory follicles (POFs) up to 90 days postlaying to determine breeding propensity and clutch size in Mallards (Anas platyrhynchos). During 2002–2003, we examined ovaries from 48 postbreeding female Mallards (24 captive, wild-strain and 24 radio-marked, free-ranging) with known laying histories at periods ranging from 30 to 90 days postlaying. We found that POFs remained identifiable macroscopically (at 4.6–16× magnification) in all females (24 of 24) collected 30–60 days postlaying and in 79% (19 of 24) of females collected 61–90 days postlaying. Using microscopic examination (20–80× magnification), we found that ≥1 POFs remained identifiable in all cross-sectioned ovaries collected from females up to 90 days postlaying. We found a weak correlation between macroscopic POF counts and observed clutch size for females collected 30–60 days postlaying, but not for females collected 61–90 days postlaying. Blind tests using ovaries from known nonbreeding females confirmed that experienced, independent observers correctly distinguished breeding from nonbreeding females 100% of the time up to 60 days postlaying using macroscopic techniques and up to 90 days postlaying using microscopic techniques. These findings indicate that POFs provide a viable method to estimate breeding propensity in free-ranging Mallard populations and potentially other waterfowl species.

POSTOVULATORY FOLLICLES IN MALLARDS: IMPLICATIONS FOR ESTIMATES OF BREEDING PROPENSITY

Abstract ABSTRACT We evaluated the use of postovulatory follicles (POFs) up to 90 days postlaying to determine breeding propensity and clutch size in Mallards (Anas platyrhynchos). During 2002–2003, we examined ovaries from 48 postbreeding female Mallards (24 captive, wild-strain and 24 radio-marked, free-ranging) with known laying histories at periods ranging from 30 to 90 days postlaying. We found that POFs remained identifiable macroscopically (at 4.6–16× magnification) in all females (24 of 24) collected 30–60 days postlaying and in 79% (19 of 24) of females collected 61–90 days postlaying. Using microscopic examination (20–80× magnification), we found that ≥1 POFs remained identifiable in all cross-sectioned ovaries collected from females up to 90 days postlaying. We found a weak correlation between macroscopic POF counts and observed clutch size for females collected 30–60 days postlaying, but not for females collected 61–90 days postlaying. Blind tests using ovaries from known nonbreeding females c...

Mark-recapture and behavioral ecology: a case study of Cliff Swallows

Mark–recapture and the statistical analysis methods associated with it offer great potential for investigating fitness components associated with particular behavioral traits. However, few behavioral ecologists have used these techniques. We illustrate the insights that have come from a long–term mark–recapture study of social behavior in Cliff Swallows (Petrochelidon pyrrhonota). The number of transient swallows passing through a colony per hour increased with colony size and was responsible in part for increased rates of ectoparasite introduction from outside the group into the larger colonies. Annual survival probabilities of males engaging in extra–pair copulation attempts were lower than those of males not seen to commit extra–pair copulations, suggesting that males who engage in this behavior may be inferior individuals and that females do not benefit from copulating with them. Females engaging in intraspecific brood parasitism had higher annual survival probabilities than ones either parasitized by others or not known to be either hosts or parasites. This suggests that parasitic females are high–quality birds and that brood parasitism is an effective reproductive tactic for increasing their fitness. By estimating first–year survival of chicks, we found that a clutch size of 4 eggs is often the most productive, on average, as measured by recruitment of offspring as breeders, although birds laying the more uncommon clutch size of 5 fledge more young on average. This helps to explain the observed clutch–size distribution in which clutch size 4 is the most commonly produced.

Clutch size variation in the Nazca booby: a test of the egg quality hypothesis

In obligately siblicidal bird species, aggressive behavior by a dominant chick results in a fixed brood size of one, yet these species usually show clutch size variation between individuals. Simmons proposed that variation in clutch size in obligately siblicidal species is related to a trade-off between egg quality and egg quantity: some individuals produce a single highly hatchable egg, while others produce two small, lower quality eggs. We tested the egg quality hypothesis as an explanation for observed clutch size variation in the Nazca booby (Sula granti), an obligately siblicidal seabird. We tested the assumption that egg volume is positively correlated with hatchability and the prediction that eggs from one-egg clutches are larger than eggs from two-egg clutches. We did not find a positive relationship between egg volume and hatchability in this species. Eggs from two-egg clutches were either equivalent in volume or larger than eggs from one-egg clutches. Thus, the egg quality hypothesis was rejected as an explanation for clutch size variation in the Nazca booby. Instead, two-egg clutches appear to be favored because of the insurance value of the second-laid egg, while one-egg clutches result from food limitation. Copyright 2002.

Larval Competition, Adult Fitness, and Reproductive Strategies in the Acoustically Orienting Ormiine Homotrixa alleni (Diptera: Tachinidae)

Homotrixa alleni is a gregarious endoparasitoid fly that attacks adult male Sciarasaga quadrata (Orthoptera: Tettigoniidae) in southwestern Western Australia. Gravid female flies acoustically orient to their host's call and deposit live first-instar larvae upon or near their calling host. Up to 16 larvae may be found developing in the one host, and since only calling adult male S. quadrata are parasitized, host size and hence larval resources are essentially fixed at parasitism. This study examines parasitism by H. alleni in relation to intraspecific larval competition and adult fitness. The mean number of larvae emerging per host failed to increase significantly beyond a clutch size of four. Mean pupal weight and survival to the adult stage decreased linearly with increasing clutch size across the entire range of clutch sizes examined. Within a clutch, heavier pupae successfully completed pupal development significantly more often than lighter pupae. Pupal weight was directly related to adult size, with adult males being significantly larger than adult females at any given pupal weight. Female body size was positively correlated with fecundity. The size distribution of emerging females was normally distributed, while the distribution of searching gravid females collected at acoustic traps in the field was significantly skewed toward larger flies, suggesting yet another fitness benefit associated with large size. Using fecundity and survival to adulthood as our measure of fitness we calculated the optimal clutch size maximizing fitness per host to be seven, which exceeds the majority of observed clutch sizes in the field. Uncertainties associated with larvae successfully entering the host following larviposition are likely to reduce clutch sizes of H. alleni below this optimum in the field.

Factors influencing the evolution of clutch size in a gregarious insect parasitoid

Summary 1. In this study we examined the clutch size of Hyssopus pallidus (Askew) (Hymenoptera, Eulophidae), a gregarious ectoparasitoid of codling moth (Lepidoptera, Tortricidae) larvae that is characterized by a host handling time of 2 days approximately. 2. Clutch size increased asymptotically with host larval size (fresh weight) to a maximum of 32 eggs. A single male developed from each clutch, brood survival was 83% and pupal size of female offspring was 0·45 mg, all independent of clutch size. 3. A comparison of the response of both experienced and naïve adult parasitoids to varying host larval size, indicated that experience was not necessary for H. pallidus to be able to accurately estimate host size. 4. Clutch size increased by 50% for medium size host larvae (30–40 mg) and 83% for large host larvae (60–76 mg) when the encounter rate was reduced from one host every second day to one every seventh day. A significantly lower clutch size was produced on the last day at the highest encounter rate for large host larvae, suggesting exhaustion of the egg supply for H. pallidus. 5. Manipulation of parasitoid clutch size on constant size host larvae did not influence sex ratio (single male) or brood survival (93%), but produced a linear trade‐off between clutch size and the size (fresh weight) of female pupae. Longevity, lifetime fecundity, total number of hosts attacked, and clutch size all increased linearly with the adult size (female pupal weight). 6. The observed clutch size in newly emerged H. pallidus is consistent with a strategy of maximizing lifetime fitness gain rather than the gain from individual hosts (Lack clutch size) despite a significant investment in the time required to attack each host.

Reproductive consequences of egg‐laying decisions in snow geese

Summary1. Even though feeding conditions typically improve over time during the laying period, clutch size decreases over the course of the nesting period in most bird species. We examined whether seasonal decrease in offspring value could explain the seasonal decline in clutch size in arctic‐nesting greater snow geese (Anser caerulescens atlanticusL.).2. Nesting was synchronized within a year, with more than 90% of the nests being initiated within about 8 days. Despite this high synchrony, there was a steep seasonal decline in clutch size in each of the 7 years of the study, from about five eggs in early clutches to three in late ones (−0·20 egg day−1).3. Late parents performed more poorly than early parents in most components of reproductive success. The relationship between laying date and nesting success was curvilinear, early and late nests having a higher failure rate than those initiated near the median. Prefledging survival decreased by about 50% over the season, although the earliest hatched goslings also tended to have a reduced survival. The postfledging survival showed the strongest seasonal decline, as survival probability of late‐hatched birds was about five times lower than in early‐hatched ones.4. Overall reproductive success showed a very steep seasonal decline as the number of young surviving to the first winter was about eight times lower in late‐nesting birds than in early‐nesting ones. Reproductive success declined slightly in the earliest‐nesting birds, suggesting a cost to nesting too early.5. The observed clutch size generally matched the clutch size that yielded the highest reproductive success for each laying date, except in earliest‐nesting birds which should have done better by slightly delaying nesting. Our data suggest that trading off additional eggs for earlier nesting to increase reproductive success is an option in geese. Consequently, the seasonal decline in clutch size may be an adaptive response to seasonally declining survival prospects of offspring.

An Experimental Analysis of Small Clutch Size in Tropical House Wrens

Trade—offs are central to life history theory, yet few studies have examined how geographic variation in trade—offs can lead to geographic variation in life history characters. I examined whether or not trade—offs for future fecundity or offspring survivorship could explain why tropical birds lay smaller clutches than their temperate relatives. I studied a tropical population (in Monteverde, Costa Rica) of the House Wren (Troglodytes aedon), a species that ranges in average clutch size from 6 in the temperate zone to 3.5 in the tropics. Three years of brood manipulation experiments showed weak effects of brood size on both future fecundity and offspring survivorship. Females that raised broods enlarged by two nestlings laid subsequent clutches, in the same breeding season, that were one—third of an egg smaller than those of females that did not raise enlarged first broods. Clutches in the year following brood manipulation were about a half an egg smaller for females raising enlarged broods than for females raising control or reduced broods. However, brood manipulation had no effect on male or female survivorship in any year of the study, despite the observation that both sexes increased their foraging rate to compensate for rearing larger broods. In two of three years, House Wrens were able to raise enlarged broods just as successfully as control and reduced broods, as measured by fledgling mass and survivorship of nestlings and fledglings. In one year, nestlings in enlarged broods fledged lighter and had lower fledgling survival than those in control or reduced broods. Predation of broods was unrelated to brood size, so food limitation appeared to be the mechanism causing the trade—off between brood number and offspring production. The pattern in tropical House Wrens is similar to that found in many studies of temperate passerines: in most years, brood sizes larger than the modal brood size appear to produce the most offspring. Thus, the same mechanism that controls clutch size in temperate birds may be at work in the tropics, but the level at which clutch size is controlled is lower in tropical birds, resulting in smaller clutches. A population model based on demographic parameters measured in the study population showed that the trade—off for offspring survivorship had a greater influence on fitness than the trade—off for future fecundity. Also, the clutch size strategy accruing the highest fitness depended on temporally varying conditions for reproduction. A strategy of laying 5—6 eggs had higher fitness than laying smaller clutches in years when conditions were favorable for reproduction, but clutch sizes of 3—4 (the observed clutch sizes in Monteverde) were most productive during less favorable years. Depending on the frequency of favorable years, House Wrens may be responding to a "ad—years effect" by lowering variance in reproductive success to maximize fitness over the long term. Alternatively, tropical birds may lay fewer eggs so that they can invest more care in each offspring, enhancing the chance that their offspring will survive and compete successfully in social contests for breeding territories. This offspring—quality hypothesis is supported by the observation that tropical House Wrens devote more time to the different stages of the reproductive cycle than do temperate House Wrens.

A within-clutch trade-off between egg production and rearing in birds

Here we compare the chick-rearing capacity of common terns Sterna hirundo that were given an additional chick at hatching (incurring rearing costs but no production costs), an additional egg at the start of incubation (incurring incubation and rearing costs) and made to lay an additional egg (incurring laying, incubation and rearing costs). The brood enlargements were within the natural range of brood sizes (i.e. two increased to three). Only those pairs that were given a free chick reared a significantly larger brood compared with unmanipulated controls. The results show for the first time a within-clutch trade-off between producing and rearing extra young. Although there was no effect on egg or hatchling size as a result of the increased egg production, parents incurring the full costs of producing extra young subsequently showed reduced chick provisioning, growth and survival. These results cast doubts on the interpretation of previous brood enlargement experiments as providing empirical evidence that observed clutch sizes are often less than the Lack clutch size, as such experiments have failed to include production costs.

Clutch Size in a Larval-Pupal Endoparasitoid: Consequences for Fitness

1. Aphaereta minuta (Hymenoptera: Braconidae) is a gregarious larval-pupal endoparasitoid of many Diptera species. Several larval instars can be parasitized and the size differences between host species can be considerable. After parasitization, however, the host larva continues to grow and the parasitoid's eggs hatch after the host pupates. We question whether this delay between the moment of oviposition and that of resource availability for offspring development hampered the ovipositing female in making optimal clutch size decisions. 2. Using an optimality approach we analysed the relationship between clutch size (number of eggs) and fitness in different instars of the host Delia antiqua in the laboratory. 3. Clutch size was artificially manipulated, and the relationship between clutch size and fitness was quantified using the following parameters: offspring egg to adult survival, sex ratio of emerging adults and size of emerging daughters (since size and number of eggs are positively correlated, size is a measure of fecundity). 4. Survival to adult stage was slightly positively correlated with clutch size in first instar host larvae and negatively correlated with clutch size in second instar host larvae. Sex ratio (proportion daughters) increased with increasing clutch size. The size of both males and females on emergence was negatively related to clutch size, and more strongly to the number of emerging adults. 5. The calculated Lack clutch size (whereby fitness is maximized per clutch) increased with larval host stage, as did the observed clutch size. 6. For each instar the observed clutch size was lower than the calculated Lack clutch size. We argue that under natural conditions females are selected to lay a clutch lower than the Lack clutch size.

The costs of reproduction in tree swallows (Tachycineta bicolor)

We investigated the effect of brood size on nestling growth and survival, parental survival, and future fecundity in tree swallows (Tachycineta bicolor) over a 4-year period (1987–1990) in an effort to understand whether reproductive trade-offs limit clutch size in birds. In addition to examining naturally varying brood sizes in a population on Kent Island, New Brunswick, Canada, we experimentally modified brood sizes, increasing or decreasing the reproductive burdens of females by two offspring. Unlike previous studies, broods of the same females were enlarged or reduced in up to 3 successive years in a search for evidence of cumulative costs of reproduction that might go undetected by a single brood manipulation. Neither observation nor experiment supported the existence of a trade-off between offspring quality and quantity, in contrast with the predictions of life-history theory. Nestling wing length, mass, and tarsus length were unrelated to brood size. Although differences between means were in the direction predicted, few differences were statistically significant, despite large sample sizes. Nestlings from small broods were no more likely to return as breeding adults than nestlings from large broods, but return rates of both groups were very low. Parental return rates were also independent of brood size, and there was no evidence of a negative effect of brood size on future fecundity (laying date, clutch size). Reproductive success, nestling size, and survival did not differ between treatments for females whose broods were manipulated in successive years. Within the range of brood sizes observed in this study, the life-history costs of feeding one or two additional nestlings in tree swallows appear to be slight and cannot explain observed clutch sizes. Costs not measured in this study, such as the production of eggs or postfledging parental care, may be more important in limiting clutch size in birds.

Predicting Clutch Size of the House Wren with the Murray-Nolan Equation

Although there have been many hypotheses and much research concerning factors that influence clutch size, there have been few attempts to predict clutch size. Murray (1979) suggested that females in a population should lay as few eggs per clutch as required to replace themselves and therefore maximize their survivorship and lifetime reproduction. Murray and Nolan (1989) used this idea to develop an equation that predicts clutch size for birds based on female survivorship and other demographic data. When such data for Prairie Warblers (Dendroica discolor) and Florida Scrub Jays (Aphelocoma c. coerulescens) were used in this equation, close agreement was found between predicted and observed clutch sizes (Murray and Nolan 1989, Murray et al. 1989). To test the Murray-Nolan equation as a predictor of clutch size in House Wrens (Troglodytes aedon), I used demographic data extracted from the field notes of the Baldwin Bird Research Laboratory. Female House Wrens, unlike female Prairie Warblers and Florida Scrub Jays, often rear > 1 brood during a season, so this paper represents an extension of earlier tests of the Murray-Nolan equation to a predominantly double-brooded species. Between 1919 and 1940, S. P. Baldwin, S. C. Kendeigh, and others at the Baldwin Bird Research Laboratory, Gates Mills, Ohio, studied breeding and survivorship of House Wrens. Field notes of this research were made available to me from the library of the Bird Division, Museum of Zoology, University of Michigan. I compiled and analyzed the records from the notebooks for 1926-1937, the 12 yr of most active research. For each nest I recorded the band numbers of both male and female adults, the number of eggs laid, the number of eggs that hatched, the number of hatchlings that successfully left the nest (fledged), and the band numbers of those hatchlings that fledged. Each year, almost all breeding birds were captured at the box and banded or, if previously banded, had their band numbers recorded. Breeding birds not captured were usually those whose clutches failed shortly after laying began. Breeding females were assigned an age of 1 yr if they had not been caught previously at the study sites (see Kendeigh and Baldwin 1937) or if they had been banded as nestlings during the previous year. Only 69 of 4,188 (1.7%) females banded as nestlings returned to breed on the study site; 64 of the 69 (93%)

THE EVOLUTION OF CLUTCH SIZE. II. A TEST OF THE MURRAY-NOLAN EQUATION.

We test the Murray-Nolan equation for calculating clutch sizes of birds with demographic data from a long-term study of the Florida Scrub Jay (Aphelocoma c. coerulescens). The predicted value, 3.43, is very close to the observed clutch size, 3.33. This result provides further support for the equation's being an adequate description of the relationship among the factors affecting the clutch size of birds.

Demographic Features of a Lapland Longspur Population near Barrow, Alaska

-Breeding density, clutch size, hatching and fledging success, and survival of adult Lapland Longspurs (Calcarius lapponicus) were monitored over a 7-year period near the Naval Arctic Research Laboratory, Barrow, Alaska. Nesting begins as soon as the tundra starts to clear of snow and appears to be timed so that the young of the year reach independence before the end of adult insect emergence. Arthropod prey become abruptly scarce after the period of insect emergence and thus probability of success for late broods is low. Time of nesting is also discussed in relation to factors of predation and timing of molt. At least 42.9% of males and 45.4% of females survived the next season after 1 year of age. Maximum longevity observed for both sexes is 6 years. Mean clutch size for all years investigated was 5.06 eggs per clutch. Clutch size showed both yearly and seasonal variation. The data indicate that timing of nesting is the chief factor in the observed clutch size differences, further modified by habitat quality, second nesting attempts, and probably age of female. Over a 4-year period the longspur breeding population on a 17-ha study plot declined from 15 to 2 breeding females. Life-table analysis indicates that low fledgling success for 3 consecutive years apparently was the main cause of this decline. Overall mean egg success was 44.0%, but close to 60% in 4 years considered more normal. The low egg success was due to 3 consecutive years of high predation, which may have been related to the disrupted lemming cycle of the Barrow region.-Museum of Vertebrate Zoology, University of California, Berkeley, California 94720. Present address of senior author: Patuxent Wildlife Research Center, Laurel, Maryland 20811. Accepted 9 December 1975. THE increase in fieldwork in the Arctic in the 1950's and 1960's has made it possible to examine various aspects of the demography of passerine birds there (Ricklefs 1969, 1973; Hussell 1972). Even so, information on density, clutch size, and hatching success of arctic birds is generally scarce, and only a few recent studies (e.g.Williamson 1968, Williamson and Emison 1971, Hussell 1972) had been carried out for more than one season. The demographic data presented here represent part of an investigation dealing with the role of the Lapland Longspur (Calcarius lapponicus) in the arctic coastal tundra ecosystem of northern Alaska. Data on clutch size, hatching and fledging success, and survival of adults, which span a seven-year period (1967-1973), are analyzed by life-table techniques. During the course of the study longspur breeding density decreased drastically at Barrow. The life-table analysis helps explain the observed decline. Some data pertinent to various aspects of this paper are also drawn from work on the Lapland Longspur by F. A. Pitelka during the period 1951-1969. Other parts of this study, dealing with diet, activity patterns, molt, and bioenergetics will be published elsewhere. Background information on coastal tundra as a habitat is available in Britton (1957) and Wiggins and Thomas (1962). The total ecological setting is compactly summarized by Bunnell et al. (1975).

An Unusual House Sparrow Roost

The effect of year, which includes effects of date and density, was based on the value for Marley Wood + 0.2 in 1960, and on 1/ Marley + 2/3 Great Wood for 1961-69. The effects are given in table A3. Habitat. Differences in habitat type, classified as good and bad, within each produced a small, but only just significant, difference (mean weighted difference 0.17 -0.08 eggs), which was not considered worthwhile applying as a correction factor in the present analysis. The classification into good and bad was made subjectively by the authors, and is not comparable to that of Krebs (1970), who used consistently occurring extreme values for clutch size as criteria for habitat quality and ignored middle values. Corrections applied. To allow for the effects of area and year the effects calculated above in tables A2 and A3 were substracted from each observed clutch size. To allow for the effect of the age of the female, 0.3 eggs was added to the clutch of a yearling and subtracted from that of an older bird.